Computer graphics systems represent graphical primitives as pixel elements of a display. Aliasing refers to the visual artifacts, such as stair-stepping of surface edges (sometimes known as “jaggies”), that occur when an image is sampled at a spatial sample frequency that is too low.
A variety of anti-aliasing techniques exist to reduce visual artifacts. One example is supersampling, in which an image is sampled more than once per pixel grid cell and the samples are filtered. For example, in supersampling the contribution of each sample in a pixel may be weighted to determine attributes of the pixel, such as the pixel color. Another example of an anti-aliasing technique is multisampling. As objects are rendered in a multisampling system, a single color is typically computed per primitive and used for all subpixel samples covered by the primitive. Additional background information on anti-aliasing techniques used in graphics processing units (GPUs) may be found in several patents issued to the Nvidia Corporation of Santa Clara, Calif. such as U.S. Pat. No. 6,452,595, “Integrated graphics processing unit with anti-aliasing,” U.S. Pat. No. 6,720,975, “Super-sampling and multi-sampling system and method for anti-aliasing,” and U.S. Pat. No. 6,469,707, “Method for efficiently rendering color information for a pixel in a computer system,” the contents of each of which are hereby incorporated by reference.
In both supersampling and multisampling the quality of the anti-aliasing tends to improve as the number of samples per partially covered pixel increases. For example, increasing the number of samples per pixel from four samples (“4×” sampling) to sixteen (“16×” sampling) would be expected to reduce visual artifacts. However, as the number of samples per pixel increases more sample data must be generated, stored, transported, and processed. Consequently, the required memory resources, computing resources, and memory bandwidth may increase as the number of samples per pixel is increased. As a result, the cost and complexity of a graphics system tends to increase as the number of samples used for anti-aliasing increases.
Therefore, what is desired is an improved apparatus, system, and method for anti-aliasing that achieves many of the same benefits associated with an increase in the number of samples per pixel but without the corresponding increase in cost and complexity associated with conventional anti-aliasing techniques.